Photoreduction and ketone-sensitized reduction of alkaloids

Photoreduction and Ketone-sensitized Reduction of Alkaloids

Helmut Gorner1, Zsombor Miskolczy2, Monika Megyesi2 and Laszlo Biczok*2

1Max-Planck-Institut fur Bioanorganische Chemie, Mulheim an der Ruhr, Germany2Chemical Research Center, Hungarian Academy of Sciences, Budapest, Hungary

Received 16 November 2010, accepted 10 December 2010, DOI: 10.1111/j.1751-1097.2010.00880.x

ABSTRACT

The photoprocesses of berberine, palmatine, coralyne, sanguin-

arine, flavopereirine and ellipticine were studied in several

solvents. The quantum yields FD of singlet molecular oxygen

formation of berberine, palmatine and sanguinarine are moder-

ate in dichloromethane (0.2–0.6) and much smaller in acetonitrile

or trifluoroethanol. For the other alkaloids examined, FD is

rather independent of solvent polarity. The direct and ketone-

sensitized photolysis, using steady-state irradiation at 313 nm or

248 ⁄ 308 nm laser pulses, was studied by absorption and fluo-

rescence spectroscopy. Thereby, radicals were observed yielding

eventually dihydro derivatives as major products, which are

thermally back-converted on admission of oxygen. The quantum

yield of conversion of alkaloids to dihydroalkaloids is enhanced

in the presence of triethylamine. The reaction in the presence of

ketones and electron or H-atom donors has a quantum yield of

close to unity.

INTRODUCTION

Berberine, a clinically important natural isoquinoline deriva-tive and the major alkaloid in Goldenseal (Hydrastis canad-

ensis L.), has been the subject of several thermodynamic,photophysical and photochemical studies (1–23). Similarstudies have been reported for palmatine (4,5) and other

related alkaloids, such as coralyne (24–29), sanguinarine (30–33), ellipticine (34–39) and flavopereirine (40). The molecularstructures for these compounds are presented in Scheme 1.Knowledge of the radiationless processes of natural isoquin-

oline alkaloids, e.g. the quantum yield of intersystem crossing(Fisc), is scarce. The formation of singlet molecular oxygen,O2(

1Dg), has been detected for berberine, palmatine and

sanguinarine (4,5,30). The photochemistry and phototoxicityof berberine and palmatine have been examined (7,8). A laserflash photolysis study revealed the properties of berberine in

the triplet-excited state (15).The size of the macrocycle is the dominant factor deter-

mining the binding constant between berberine and p-sulfo-nated calixarenes, and the largest effect of inclusion on the

fluorescence quantum yield (Ff) is found at pH 2 (7). Berberinehas been applied as a sensitive fluorescent probe for bile saltaggregates (8) and for the study of the supramolecular complex

formation of ionic liquids (11). Some fluorescence features of

alkaloids have recently been reported (7–11,14). Upon addi-tion of cucurbit[7]uril (CB7), the fluorescence intensity ofberberine in water increases up to 500-fold (10). The fluores-cence enhancement has been ascribed to the slower relaxation

in the rather nonpolar cavity of the host. Isoquinolinealkaloids bind to macromolecules, such as DNA, polynucleicacids, RNA and serum albumin (12,24,25). Efficient photoin-

duced DNA damage by coralyne has been reported (26).Complexation may hinder bimolecular reactions therebyprotecting the alkaloid against photodecomposition, or may

lead to efficient excited state quenching if electron transfer orenergy transfer to the ligand is possible.

In this work, we study the photoprocesses and ketone-sensitized reactions of selected alkaloids, such as berberine,

palmatine, coralyne, sanguinarine, flavopereirine and ellipti-cine. One goal is a better characterization of the tripletproperties of the alkaloids in nonaqueous solution. The second

goal is the examination of the new finding that the majorphotoproducts are dihydroalkaloids, which revert back to thecorresponding alkaloids on the exposure of oxygen. We

demonstrate that the dihydroproducts can be most efficientlyproduced by the reaction of alkaloids with photogeneratedradicals. For this purpose, acetone, acetophenone and benzo-

phenone with 2-propanol or triethylamine (TEA) were used asadditives. We demonstrate that the quantum yield of ketone-sensitized reduction of alkaloids in acetonitrile approachesunity under optimized conditions.

MATERIALS AND METHODS

2,2,2-Trifluoroethanol (TFE, EGA), the alkaloids, berberine (Sigma),coralyne (Acros), palmatine (Aldrich) and sanguinarine (Fluka) aschloride salts, flavopereirine perchlorate (ChromaDex), and ellipticine(Fluka) were used as received. TEA was distilled prior to use, whereassolvents (Merck) of the purest spectroscopic quality were employedwithout further purification. The absorption spectra were monitoredon a spectrophotometer (HP, 8453). The molar absorption coefficientsof berberine, sanguinarine and flavopereirine in (m)ethanol weretaken as e420 = 5.0 · 103 (1), e320 = 3.1 · 104 (31) and e390 =1.4 · 104 MM

)1 cm)1 (40); for coralyne and ellipticine, we usede420 = 1.5 · 105 and e295 = 6 · 105 MM

)1 cm)1. The quantum yieldFred of photoreduction was obtained using the aberchrome actinom-eter (41); the experimental error is ±30%. A spectrofluorimeter (Cary,Eclipse) was employed to measure the fluorescence spectra. Thefluorescence yields were determined using rhodamine 101 as referenceand optically matched solutions. An excimer laser (Lambda Physik,EMG 200) pulse width of 20 ns and energy <30 mJ was used forexcitation at 308 nm. In a few cases, kexc = 248 nm was used.Optically matched solutions, typically with absorbances at the excita-tion wavelength of 0.2–2 (per 1 cm), were used in the measurements ofrelative quantum yields. The absorption signals were recorded with

*Corresponding author email: [email protected] (Laszlo Biczok)� 2011 The AuthorsPhotochemistry and Photobiology� 2011 The American Society of Photobiology 0031-8655/11

Photochemistry and Photobiology, 2011, 87: 284–291

284

two digitizers (Tektronix 7912AD and 390AD). Phosphorescence ofsinglet molecular oxygen at 1269 nm was detected after the pulse usinga cooled Ge detector (North Coast, EO 817FP), a silicon filter, aninterference filter and an amplifier (Comlinear, CLC-103) as describedelsewhere (42). The signal, which can be overlapped by fluorescenceand ⁄ or scatter, was extrapolated to the end of the 20-ns pulse (ID). At afixed laser intensity, ID showed a linear dependence on the absorbedenergy, being proportional to (1–10)A), and deviation from thelinearity was observed only at higher intensities. The quantum yieldFD was obtained from ID values using optically matched solutions(A308 = 0.8) and 2-acetonaphthone in benzene as reference,FD = 0.85. A correction has to be applied to account for the lowerrate constants for radiative deactivation in dichloromethane, acetoni-trile and other solvents relative to that in benzene (42). Themeasurements refer to 297 K.

RESULTS AND DISCUSSION

Direct photolysis

The absorption spectrum of berberine in argon-saturateddichloromethane with maxima at ka = 280, 355 and 440 nm

changes as a function of the time of irradiation at 313 nm orthe number of pulses of 308 nm light (inset to Fig. 1).Berberine and palmatine were found to be rather photostable

in acetonitrile or aqueous solution, whereas somewhat moreefficient decomposition occurred in dichloromethane. As seenin Fig. 1, the spectral changes were strongly enhanced in thepresence of TEA due mainly to photoreduction, and slightly

more rapid reaction is observed in acetonitrile. Scheme 2accounts for the photoprocesses. The triplet state of thealkaloid (3*A+) is populated by steps 1 and 2. Quenching by

oxygen in step 3 competes with the reaction of the tripletexcited alkaloid with hydrogen or electron donors (DH2).As a measure of the spectral changes, we define the ratio

of absorbances after maximum conversion and prior to

irradiation Ared ⁄A0 at the wavelength of maximum change(ka), e.g. 350 nm for berberine or palmatine. The ka andAred ⁄A0

values of the alkaloids examined are compiled in Table 1.

Figure 2 shows the considerable photostability diminutionof coralyne upon addition of TEA and acetone with TEA indeoxygenated acetonitrile. The photoproduct is efficiently

converted back to the starting compound staying 2 h in air-saturated solution (inset). After purging with argon, thephotoreduction can be repeated. In the case of flavopereirine,

the formation of a product with an absorption peak atkp = 300 nm as well as the appearance of isosbestic pointswere observed (Fig. 3).

The increase of the TEA concentration from 5 to 50 mMM

does not affect the reaction rate indicating that 5 mMM TEA isenough to make the electron transfer to triplet flavopereirinethe dominant excited state deactivation pathway. The quan-

tum yield Fred of alkaloid reduction is significantly enhancedupon addition of 10 mMM TEA, and the measured Fred valuesare summarized in Table 2. Fred is larger in dichloromethane

than in acetonitrile for berberine and palmitine because of the

N

OO

OCH3

H3CO +

N

CH3

H3CO

H3CO

OCH3

OCH3

+

N

OCH3

H3CO +

OCH3

OCH3

N+

O

O

OO

CH3

NH

N+

NH

N

CH3

CH3

Berberine Palmatine

Coralyne Sanguinarine

Flavopereirine EllipticineScheme 1.

0 200 400 600

1

2

300 350 400 4500

1

2

1'22'

1

A

λ (nm)

A

Time (s)Figure 1. Absorbance of berberine at 350 (open symbols) and 380 nm(full symbols) in argon-saturated dichloromethane as a function of thetime of irradiation at 313 nm (squares) as well as in the presence of10 mMM triethylamine. Solvents: dichloromethane (circles) and acetoni-trile (triangles). Inset: spectra in neat acetonitrile and dichloromethane(A = 0.5 upward shifted) prior to (1 and 2, respectively) and after500 s irradiation (1¢, 2¢).

A+1

-D

hν A+ A+ AH+*3 +DH2 -DH• •

AH2+

2x -A+

(1)

(2)

(3) (4)(5)(6)

+DH•+O2

*

Scheme 2.

Table 1. Absorption maxima of alkaloids and photoproduct as well asthe relative absorbance changes upon irradiation*.

Alkaloid ka (nm) Ared ⁄A0 kp (nm)

Berberine† 350, 430 0.4 370Coralyne 310, 425 0.4 ⁄ 0.1 370Flavopereirine 345, 390 0.5 300

*In argon-saturated acetonitrile solution, kirr = 313 nm; Ared ⁄A0 referto maximum changes. †Very similar values as with palmatine.

Photochemistry and Photobiology, 2011, 87 285

larger quantum yield Fisc of their triplet formation in less polar

medium (vide infra). Fred grows when 0.1 MM TEA is added inacetonitrile indicating the contribution of the reaction from thesinglet-excited state of alkaloids.

Ketone-sensitized photolysis

The photochemical transformation of alkaloids is significantlyaccelerated when the solutions contained ketone and TEA orketone and 2-propanol. The quantum yields FK of ketone-

sensitized reduction of alkaloids in acetonitrile are rather low,but strongly enhanced on addition of small amounts of TEAor 2-propanol. In fact, when oxygen was carefully avoided, FK

was found to be close to unity (Table 3). The ketone-sensitized

photolysis at 313 or 308 nm brings about the same spectralchanges above 320 nm and leads to the same photoproduct asfound by direct irradiation. The product is identified asdihydroalkaloid, which is selectively formed at low conversion.

This is supported by the finding that the photoproduct isthermally back-converted into the starting compound uponadmission of oxygen as shown in the insets of Figs. 2–4. The

spectral alterations barely differed for berberine and palma-tine. The absorption of these compounds vanished in the lowenergy domain of the spectrum (inset to Fig. 4), and a new

blue-shifted band appeared with a maximum at 370 nm, which

0 200 4000.5

1.0

1.5

2.0

300 350 4000

1

2

43

2

1

A

λ (nm)

A

Time (s)Figure 2. Absorbance of coralyne at 320 nm in argon-saturatedacetonitrile as a function of the time of irradiation at 313 nm (h), inthe presence of 5 mMM triethylamine (TEA) (d), 50 mMM TEA (m) and0.5 MM acetone plus 1 mMM TEA (s). Inset: absorption spectra in thepresence of 0.02 MM TEA prior to (1) and at 10 s irradiation (2), 2 hafter admission of air to the photoproduct (3), argon-saturationfollowed by 10 s irradiation again (4).

0 200 400

0.5

1.0

300 4000

1

32

1A

λ (nm)

A

Time (s)Figure 3. Absorbance of flavopereirine at 370 nm in argon-saturatedacetonitrile vs time of irradiation at 313 nm (h), in the presence of5 mMM TEA (d), 50 mMM TEA (m) and 0.5 MM acetone plus 0.2 MM 2-propanol (s). Inset: absorption spectra in the presence of 0.5 MM

acetone plus 0.2 MM 2-propanol prior to (1) and at 20 s irradiation (2)and 2 h after admission of air to the photoproduct (3).

Table 2. Quantum yield Fred of reduction of alkaloids*.

Alkaloid CH2Cl2 + TEA† CH3CN + TEA†

Berberine 0.08 0.02 (0.03)‡Palmatine 0.07 0.02Coralyne 0.05 0.05 (0.06)Sanguinarine 0.06 0.02Flavopereirine 0.04 0.04 (0.05)

*In argon-saturated solution, kirr = 313 nm. † [TEA] = 0.01 MM.‡Values in parentheses: [TEA] = 0.1 MM. TEA = triethylamine.

0 10 20 30

1

2

300 4000

1

213

24

5

A

λ (nm)0

A

Time (s)Figure 4. Absorbance of palmatine at 350 nm (d) and 440 nm (s) inargon-saturated acetonitrile in the presence of 0.5 MM acetone plus 1 MM

2-propanol as a function of the time of irradiation at 313 nm. Insets:absorption spectra prior to and after 20 s irradiation and 24 h afteradmission of air the photoproduct, 1–3, respectively, second cycle,4 and 5.

Table 3. Quantum yield FK of ketone-sensitized reduction of alka-loids*.

AlkaloidAcetone +

TEAAcetone +2-propanol

Benzophenone +2-propanol

Berberine 0.8 [0.7]† 0.8 0.7Palmatine 0.8 0.8 (<0.01)‡ 0.7 (<0.02)Coralyne 0.8 0.8 0.8Sanguinarine 0.6Flavopereirine 0.8 0.8

*In argon-saturated acetonitrile solution, kirr = 313 nm, 0.01 MM TEA,1 MM 2-propanol. †Value in brackets: kexc = 308 nm. ‡Values inparentheses: air-saturated. TEA = triethylamine.

286 Helmut Gorner et al.

corresponded to the spectrum of dihydroberberine available inthe literature (43,44). It is typical for dihydroprotoberberinealkaloids that the first absorption peak is located in the 350–375 nm range (45). The formation of these products has also

been found by polarographic reduction of the C = N bond ofberberine (46). The first step of this reaction is an electrontransfer to the isoquinoline moiety leading to radical possess-

ing an unpaired electron at eight position (Scheme 4).MOPAC calculations from Inbaraj and coworkers showedthat the spin density at eight site is high and a spin trap

molecule adds to this position (4). Further electrochemicalreduction of the radical produces dihydroberberine, which canbe oxidized by H2O2 back to berberine (46). Our results

demonstrate that reduction of alkaloids to dihydro derivativecan also be performed via photochemical reactions, and H2O2

is not needed for the back oxidation; the process can beaccomplished with oxygen.

Fluorescence

The isoquinoline alkaloids exhibit fluorescence, some havebeen reported (7–11,14) and the excitation spectrum coincides

with the absorption spectrum. Therefore, the conversion ofalkaloids to dihydro derivatives can also be followed byfluorescence spectroscopy. In most cases, Ff of the oxidized

form is larger than that of the dihydro form. This is also thecase for berberine and palmatine in dichloromethane, but inacetonitrile the effect is opposite. A main reason is the decreaseof Ff of these alkaloids with solvent polarity (14). As a

representative example, Fig. 5 displays the excitation andfluorescence spectra of palmatine in the presence of 1 MM

acetone and 0.2 MM 2-propanol prior to and after irradiation

as well as after admission of air. Moreover, the ratio of thefluorescence intensity at maximum conversion and prior toirradiation, Ired ⁄ I0 is plotted as a function of the irradiation

time. To achieve selective excitation of the starting compound,the fluorescence was recorded using 450 nm excitation wave-length. The rate of sensitized photoreduction of palmatine

barely differs in dichloromethane and acetonitrile and thefluorescence of palmatine almost completely vanishes after100 s irradiation at 313 nm due to the practically completeconversion into dihydropalmatine. The recovery of palmatine

by the dark reaction of the product with oxygen is evi-denced by the appearance of the original fluorescence after 1 h.

Phosphorescence of singlet molecular oxygen

Triplet excited alkaloids transfer their energy to oxygenmolecules producing O2(

1Dg) singlet excited oxygen molecules.Since the energy transfer is very efficient, the quantum yield of

singlet molecular oxygen formation (FD) provides an estimatefor the quantum yield of triplet alkaloid formation. TheO2(

1Dg) phosphorescence appears within the laser pulse orafter some scattering signal in the 1–3 ls range and decays

with a lifetime of 30–90 ls. Plots of the end-of-pulse signalintensities as a function of laser energy are initially linear. Theslope is proportional to the quantum yield of singlet molecular

oxygen formation (FD). The largest FD of 0.6 was found forflavopereirine in dichloromethane. In acetonitrile and TFE, FDis relatively large for coralyne, flavopereirine and ellipticine,

whereas it is small for berberine and palmatine (Table 4). Thediminution of FD with the increase of solvent polarity forberberine and palmatine is due to the considerable acceleration

of internal conversion, which lessens the quantum yield oftriplet formation. For berberine in dichloromethane, thequantum yield measured in this work (FD = 0.28) is betweenthe literature values of 0.25 (30) and 0.34 (4,5). Our results for

palmatine in dichloromethane (FD = 0.18) and acetonitrile(FD = 0.07) agree with the literature values of 0.20 and 0.11,respectively (5). For sanguinarine, we obtained FD = 0.04 in

acetonitrile and FD = 0.23 in dichloromethane. Taking ETN

as a measure of solvent polarity (47), considerable diminutionof FD was found in more polar media in the case of berberine

and palmatine (Table 4). In contrast, the FD values of

0 100

0.5

1.0

400 5000.0

0.5

1.0

1

3

2

3

2

1If

λ (nm)

0

Ired

/I0

Time (s)Figure 5. Plots of the fluorescence intensity at 550 nm(kexc = 450 nm) as a function of the time of irradiation at 313 nmof palmatine in argon-saturated dichloromethane (m) and acetonitrile(s) in the presence of 1 MM acetone and 0.2 MM 2-propanol. Inset:fluorescence excitation (left, kf = 550 nm) and emission (right,kexc = 450 nm) spectra in dichloromethane prior to (1) and at 90 s(2) and 1 h after admission of air to the photoproduct, 1–3,respectively.

Table 4. Quantum yield FD of O2(1Dg) formation in alkaloid solutions*.

Solvent ETN† Berberine Palmatine Coralyne Flavopereirine Ellipticine

Benzene 0.10 0.45THF 0.21 0.30 0.30 0.40 0.30Dichloromethane 0.31 0.28 0.30 0.42 0.6 0.32Acetonitrile 0.46 0.08 0.07 0.39 0.5 0.28Ethanol 0.65 0.04 0.03TFE 0.9 0.40 0.3

*In argon-saturated solution, kexc = 308 nm. †Polarity, see ref. [47]. THF = tetrahydrofuran; TFE = 2,2,2-trifluoroethanol.


coralyne, flavopereirine and ellipticine barely differed indichloromethane and acetonitrile because of the small solventeffect on the rate of photophysical processes.

Photophysics of alkaloids

The various alkaloids have been studied by spectroscopicmeans (1–40,48–51). They are salts, except for ellipticine, andare present as cations in solvents of sufficiently large polarity,

whereas ion pair formation occurs in dichloromethane. In thissolvent, K = 1.5 · 105 MM

)1 has been reported for the equilib-rium constant of the ion pairing of berberine chloride (9). Such

a process decreases the rate constant of fluorescence emissionand accelerates internal conversion (9). Therefore, it cannotexplain the high fluorescence quantum yield of berberine in

dichloromethane and its significant diminution with increasingsolvent polarity (13,14).

The relatively efficient triplet formation in dichloromethane(Table 4) is accompanied by a long fluorescence lifetime of

14.3 ns. Both T–T absorption and FD measurements show themarked diminution of the quantum yield Fisc of tripletberberine formation as the polarity of the medium grows.

The parallel shortening of the fluorescence lifetime (1.2 ns inCH3CN, 0.7 ns in CH3OH and <0.1 ns in H2O) proves thatthe environment-sensitive fluorescent properties of berberine

do not originate from the alteration of the rate of intersystemcrossing but the considerable acceleration of the internalconversion in polar medium causes this effect. The largerphotostability of berberine and palmatine in polar solvents is

due to the efficient internal conversion. The photophysicalproperties of palmatine and berberine differ only slightlybecause of the structural similarity of these alkaloids.

For ellipticine in benzene, the sum of the quantum yield ofsinglet oxygen formation (FD = 0.45 in Table 4) and fluores-cence (Ff = 0.40) is close to unity indicating that intersystem

crossing is the dominant radiationless deactivation pathwayfrom the singlet excited state. The smaller FD = 0.28 (Table 4)and Ff = 0.34 (37) values in acetonitrile imply that internal

conversion gains in importance in polar solvent. For coralyneand flavopereirine, the photophysical characteristics are essen-tially independent of solvent.

Triplet state properties

Pulsed UV excitation of berberine in solvents of differentpolarity produces the triplet-excited species with an absorptionmaximum at 430 nm and bleaching at 330–380 nm at the pulse

end or after the fluorescence signal. Examples of the T–Tabsorption spectra are shown in Fig. 6 for berberine, Fig. 7afor coralyne and Fig. 8 for sanguinarine and ellipticine. The

triplet lifetime (sT) under argon and at low laser intensity istypically sT = 10–40 ls, whereas T–T annihilation contributesto the decay at higher intensities. For berberine in acetonitrile

Fisc = 0.08 has been reported (15). Another deactivation stepis self-quenching. The rate constant of self-quenching of tripletberberine in acetonitrile is ks = 3 · 108 MM

)1 s)1 (15). Gener-

ally, the reactivity of the triplet state of the studied alkaloids islow in water. Oxygen quenches the triplet state, the rateconstant for berberine in dichloromethane and acetonitrile iskox = (0.8–1) · 109 MM

)1 s)1. The absorbance change after the

laser flash at 460 nm (DA460) is proportional to Fisc, which isgenerally low and largest in dichloromethane.

Figure 6. Transient absorption spectra of berberine in argon-saturated(a) dichloromethane and (b) acetonitrile at 20 ns (s), 1 ls (D), 10 ls(h) and 0.1 ms (d) after the 308 nm pulse; insets: kinetics at 430 nm.

Figure 8. Transient absorption spectra of (a) sanguinarine and (b)ellipticine in argon-saturated acetonitrile at 20 ns (s), 1 ls (D), 10 ls(h), 0.1 ms (d) and 1 ms ( ) after the 248 nm pulse; insets: kinetics asindicated.

Figure 7. Transient absorption spectra in argon-saturated acetonitrileat 1 ls (D), 10 ls (h), 0.1 ms (d) and 1 ms (m) of (a) coralyne,kexc = 308 nm and (b) coralyne, benzophenone and 1 MM 2-propanol,kexc = 248 nm; insets: kinetics as indicated.


Three ketones (K), acetone, acetophenone and benzophe-none were used as sensitizers to enhance the population of thealkaloid triplet state 3*A+.

3�KþAþ ! Kþ3�Aþ ðIÞ

As an example, Fig. 9a displays the acetone-sensitized

triplet berberine formation in argon-saturated acetonitrile.Since the energy of triplet acetone is 278 kJ mol)1, efficientenergy transfer can occur to berberine, whose triplet energy is

about 220 kJ mol)1 on the basis of the onset of its phospho-rescence spectrum (4). Exothermic energy transfer takes placeeven from triplet benzophenone (240 kJ mol)1) to berberine,and the rate constant is practically diffusion controlled,

kI = 5 · 109 MM)1 s)1. This process was used to determine the

molar differential T–T absorption coefficient of berberine at430 nm (De430) in acetonitrile. From the ratio of the transient

absorbances of triplet benzophenone and triplet berberine

produced by complete energy transfer from triplet benzophe-none, De430 = 2 · 103 MM

)1 cm)1 was derived using6 · 103 MM

)1 cm)1 for the molar absorption coefficient oftriplet benzophenone at 530 nm in acetonitrile.

Radical formation and decay

A test for photoionization of berberine with the laser pulse of308 nm wavelength in ethanol or aqueous solution failed.

Solvated electrons were not detected indicating a too low yieldof electron ejection. With a shorter excitation wavelength ofkexc = 266 nm, 0.03 has been found for the quantum yield of

electron ejection at pH 7 (16).The triplet state of alkaloids can be quenched by electron

donors, such as amines. For berberine or palmatine in argon-

saturated dichloromethane and acetonitrile, the rate constantof electron transfer from TEA to the triplet-excited alkaloid isk4 = 2 · 108 MM

)1 s)1. The analogous reactions of coralyneand flavopereirine have k4 = 1 · 108 MM

)1 s)1 in acetonitrile.

In this process, a neutral alkaloid radical AÆ with theunpaired electron at eight position is formed which maybecome protonated at the heterocyclic nitrogen (AHÆ+)

depending on the experimental conditions (Scheme 4). A feweffects of radiolysis of berberine and palmatine in aqueoussolution, e.g. the properties of radical AÆ, have been reported

(44). The absorbance of aliphatic amine radicals (DH2Æ+ orDHÆ) is too low to be detected. The proposed reactionsinvolved in direct and sensitized reductions are shown inSchemes 2 and 3, respectively. The alkaloid radical can be

produced most effectively upon excitation of a ketone assensitizer in the presence of both alkaloid and TEA (DH2).Thereby, two radicals (KÆ) and DH2Æ+) are formed, which can

transform into ketyl (R2CÆOH) and DHÆ radicals (step 3¢ inScheme 3). These radicals react with A+ yielding alkaloidradical, whose disproportionation leads to dihydroproduct

(AH2+) (see steps 4¢ and 5¢ in Scheme 3). In the ketone-

sensitized reactions, not only electron donor (TEA), but also

Figure 9. Transient absorption spectra in argon-saturated acetonitrileat 20 ns (s), 1 ls (D), 10 ls (h), 0.1 ms (d) and 1 ms (m) after thepulse for (a) acetone in the presence of 0.4 mMM berberine,kexc = 308 nm, and (b) benzophenone, 0.4 mMM palmatine and 1 MM

2-propanol, kexc = 248 nm; insets: kinetics as indicated.

C=ORR C=O

RR

C=ORR

C-OHRR*1

••+DH2 -DHhν •*3 +A+

AH+ AH2+

-RRC=O 2x

step 1' step 2' step 3' step 4'

-A+

step 5'Scheme 3.

N+

HH

OMeOMe

OMeMeO

N+

HHH

OMeOMe

OMeMeO

-H+

(AH)

Dihydroberberine

Berberine (AH2+)

-H+

Radical

8•

(A )•

(AH+ )••+H•+H

Scheme 4.


hydrogen donor, such as 2-propanol can be used to generatereactive radicals. In this case, the triplet ketone abstracthydrogen from alcohol producing ketyl radicals. Examples for

such reactions are shown in Figs. 7b, 9b and 10.

Properties of dihydroalkaloids

The thermal reactions of the dihydroalkaloids (AH) withoxygen probably involve O2Æ), steps 6 and 6¢.

AHþO2 ! A� þO��2 þHþ ð6Þ

A� þO2 ! Aþ þO��2 ð60Þ

Eventually, O2 is converted into H2O2. This is in analogy

with redox transformations of appropriate molecules such asquinones or specific quinoidal dyes. Various hydroquinonesare able to revert thermally back to quinones when oxygen is

added (52,53). The photoreduction products of electron-deficient azaarenes are also dihydro-compounds, which exhibitback-oxidation (54). For 7,8-dihydrobiopterin, a related het-

erocyclic compound, oxidation in aqueous solution has beenachieved photochemically (55). Such a photoreaction was notfound for the dihydroforms of berberine or palmatine underour conditions. However, photoinduced reduction followed by

virtually complete oxidation in the dark is established forberberine or palmatine (Fig. 2, inset) and coralyne (Fig. 3,inset). Note the presence of several isosbestic points. Interest-

ingly, the corresponding isosbestic points at 290, 360 and425 nm appear upon thermal reduction of berberine withsodium borohydrate (44).

CONCLUSIONS

Triplet formation is efficient for berberine, palmatine, coralyne,

sanguinarine, flavopereirine and ellipticine in dichloromethane,while Fisc is low for berberine and palmatine in acetonitrile andvirtually no triplet is produced in aqueous solution. The high

photostability of the latter two alkaloids in polar media is dueto the rapid internal conversion from their singlet-excited state.Electron transfer from TEA to triplet alkaloids promotes

photoreduction leading to dihydroalkaloids as major photo-

products. The formation and decay of alkaloid radical can bedetected by laser flash photolysis. The rate of alkaloidreduction is significantly accelerated by the combined utiliza-tion of a ketone with TEA or alcohol as photoinitiators and the

quantum yield of close to unity can be achieved for dihydro-alkaloid formation. This type of product is back-converted toalkaloids on admission of oxygen almost quantitatively and

several reduction–oxidation cycles can be performed leading tomarked fluorescence and absorbance changes.

Acknowledgements—We thank Professor Wolfgang Lubitz for his

support and Mr. Leslie J. Currell for technical assistance. The authors

very much appreciate the support of this work by the Hungarian

Scientific Research Fund (OTKA, Grant K75015), the bilateral

program between the Deutsche Forschungsgemeinschaft and the

Hungarian Academy of Sciences, CRC HAS Nanotransport and the

KMOP-1.1.2-07 ⁄ 1-2008-0002 projects.

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Photoreduction and ketone-sensitized reduction of alkaloids